# Research background
- After two devices are connected to the same LAN with different frequencies , There may be problems of communication between the two ends .
- If you pass Socket Local area network communication , If you don't have a network, you can't find the device IP, The problem of being unable to connect , There will be a delay when the Internet goes to the server 、 The problem of inaccurate data synchronization .
- In a hybrid network scenario , In connection AP when , It can communicate point-to-point at both ends at the same time , And both ends can also access the Internet separately .
- Necessary conditions for new technology direction ： Need to support at the same time Android and iOS Both ends .
Andorid End ：WiFi-P2P
Wifi P2P (peer to peer)： Defined as Wifi Point to point , Also called Wifi Direct connection (Wifi Direct), It is Wifi Display( For screen ) The technical basis of application .
The official definition of
Use WLAN Direct connection (P2P) technology , It can make the corresponding hardware Android 4.0（API Level 14） Or later devices without an intermediate access point , adopt WLAN Direct interconnection . Use these API, You can implement support for WLAN P2P Devices that discover and connect to each other , In this way, the communication effect of high-speed connection is further than that of Bluetooth connection . For applications that need to share data between users, such as multiplayer games or photo sharing , This technology is very useful .
Other definitions
WiFi Direct It refers to allowing devices in a wireless network to connect to each other without passing through a wireless router . The principle is similar to Bluetooth Technology , This standard allows wireless devices to interconnect in a point-to-point manner . And more importantly ,WiFi Direct A technology allows WiFi The device establishes multiple wireless connections at the same time , This is a lot of new forms right now WiFi Apply the most critical technology .
WiFi Direct It's a kind of P2P Wireless Internet technology of , The network it builds is an improved one adhoc The Internet , Using wireless communication mode .
WiFi Two network modes ：Infrastructure Mode and Ad Hoc Mode.
WiFi Direct It mainly solves the connection problem of physical layer , Including device discovery and service discovery . Related technologies are NFC and DLNA.
WiFi Direct Other features ：
- Unwanted AP Or a router , Device built in “soft AP”
- The two sides of a peer-to-peer connection need only one support WiFi Direct Wireless connectivity can be achieved
- Support infrastructure networks and P2P The Internet .
- Devices can be interconnected in a one-to-one or one to many manner
- As a minimum 802.11g The agreement can be passed WiFi Direct CERTIFIED verification
- The maximum transmission distance can reach 200 rice , The maximum transmission speed is 250Mbps, Use 2.4GHz And 5GHz Two frequency bands , And it supports one-on-one , And the one to many model .
Important features
- Wifi P2P Technology doesn't access the web , But because it will use Java Socket, So you need to apply for network permission .
- Just open it Wifi that will do , No need to join any network or AP, To achieve peer-to-peer connection communication .（ notes ： In the following text demo There's validation in ）
- Support the server to send messages to the devices in the cluster .
- Wifi Direct It's just pairing between devices , After matching, the file transfer operation still needs to be done by developers themselves .
Expand
Miracast Rely on the Wi-Fi The technical items are ：
- Wi-Fi Direct, That is to say Wi-Fi P2P. It supports in the absence of AP（Access Point） Under the circumstances , Two Wi-Fi The device is directly connected and communicates .
- Wi-Fi Protected Setup： Used to help users automatically configure Wi-Fi The Internet 、 add to Wi-Fi Equipment etc. .
- 11n/WMM/WPA2： among ,11n Namely 802.11n agreement , It will 11a and 11g Provided Wi-Fi The transmission rate is from 56Mbps Upgrade to 300 even to the extent that 600Mbps.WMM yes Wi-Fi Multimedia Abbreviation , Is a real-time video and audio data QoS service . and WPA2 Meaning for Wi-Fi Protected Acess The second edition , It is mainly used to encrypt and protect the transmitted data .
Aforementioned Wi-Fi In technology , Most of the functions are implemented by hardware manufacturers . And in the Android in , Yes Miracast The most important thing is two basic technologies ：
- Wi-Fi Direct： The function consists of Android Medium WifiP2pService To manage and control .
- Wi-Fi Multimedia： For support Miracast,Android 4.2 Yes MultiMedia The system has also been modified .
Use scenarios
- Mobile phone connected to AP surf the internet , function youtube Client watching video ; The mobile phone passes through at the same time Wi-Fi Direct Connect to the TV , Put the video through Wi-Fi Direct Display Projected onto the big screen .
- Realizable through Wifi Connect , Scenarios that use data networks at the same time , such as ： Mobile phone remote control UAV at the same time , UAVs need to access remote servers to upload data .
- Now in the market ,Client form （ Customer 、 User used ） Of WiFi product （ Whether it's USB Interface 、SDIO Interface or UART Interface ） All support WiFi Direct, It's just that the user perception of this technology is not strong . In fact, there are many uses in the market WiFi Direct Underlying technology App, Fast teeth, for example 、 Wali FastPass and other smart phones use their own WiFi Modules build their own network and transfer files .
Demo Research
Android End
explain ： According to the official demo, Put it in order Andorid Studio Compileable project in , And add test cases .
Code and testing apk Please click on ：WiFiDirectDemo
Official documents ：WLAN Direct connection （ Peer to peer connection or P2P） overview
Official source link ：WiFiDirectDemo
Test the installation package ：
stay apk Under the table of contents , The file named ：wifidirectDemo.apk.
The test case ：
Before the test , Remember to turn on the phone first WiFi, Otherwise, some mobile phones may have flashback problems .
Equipment support prerequisites ： Turn on WiFI And allow access to location rights .
Here are five test scenarios ：
- The same LAN scenario ：
Two phones connected WLAN hotspot ： Such as Fiture(2.4G/5G), Under the same network .
Running results ： Two mobile phones can communicate normally , Can send and receive pictures normally .
- Two routes are separated from the same LAN ：
A mobile phone connection WLAN hotspot ： Such as Fiture-Slim(5G), The other phone is connected to Fiture（2.4G/5G）.
Running results ： Two mobile phones can communicate normally , Can send and receive pictures normally .
- AP Isolate the scene ：
A mobile phone connection 4G Network virtual A hot spot , A mobile phone connection Fiture（2.4G/5G）.
Running results ： Two mobile phones can communicate normally , Can send and receive pictures normally .
- Two phones just need to be turned on WiFi, No need to connect to any network .
Running results ： Two mobile phones can communicate normally , Can send and receive pictures normally .
- Test scenarios for development boards
Same as the above four test scenarios .
Summary ： As long as both ends are open wifi, Whether it's connected to the same network or not , Can connect and send and receive pictures .
Abnormal situation ：
Sometimes you can't find it , It's going to take a few more attempts to make the search successful . In addition, we need Rom Cancel sending invitation 、 Make sure that the paired connection pop-up window .
iOS End
Multipeer connectivity It's a way to get nearby equipment through Wi-Fi The Internet 、P2P Wi-Fi And Bluetooth personal LAN communication framework .Linked nodes can transmit information safely 、 Streams or other file resources , Not through the Internet Service .
This framework is in iOS7 Later launch , Designed to replace GameKit Under the GKPeerPickerController signal communication .
Through this framework, we can directly connect devices under the same network , Let it go directly like wechat ,qq That instant messaging effect .
Use scenarios ：
How to be in iPhone Use on mobile devices Wi-Fi Direct function
Third party open source cross platform research
Android-iOS Research on cross platform transmission scheme
https://stackoverflow.com/que...
alljoyn( At present, only Android project services-simulators,iOS End did not find )
AllJoyn, The center for Innovation led by Qualcomm （Qualcomm Innovation Center） Open source project developed , It is mainly used for short distance wireless transmission , through Wifi Or Bluetooth Technology , Positioning and point-to-point file transmission .
(Android End sum iOS Each end has a corresponding App. No open source projects found ）
FireChat, It's a special mobile phone APP, Developed by open garden company . It enables smartphones to , Bluetooth via wireless mesh network 、Wi-Fi, Or Apple's multipoint connection （Multipeer Connectivity） Peer to peer networking architecture connectivity .
- udark （Android and iOS There are open source projects on both sides ,Android The bottom layer of the end is not used WiFi-P2P Interface , With Bluetooth and NSD Mainly , The document interface at both ends is not perfect ）
Mobile peer-to-peer mesh networking library.Integrates into iOS and Android apps and works over Wi-Fi and Bluetooth.
Focus on udark Conduct Demo verification , After verification, the following problems are found ：
Android End ：
You can see the underlying source code （ Only through implementation 'io.underdark:underdark:1.0.12' see jar package ）, The initial judgment of the underlying code is based on Bluetooth and WiFI(NSD) Mixed mode implementation . But the connection succeeded or failed , When looking for reasons , Because there is no document interface to explain the location problem .
iOS End ： There is an adaptation system version on the real machine 13 And the above adaptation problems , At present, only in iOS Run through the simulator Demo. Code is not open source , Can't see the connection principle , Can't locate the problem .
file ：
There is no interface documentation at both ends , Open source authors can't get further contact .
Conclusion ：
The open source project cannot be used in the project for the time being , Other directions need to be considered .
The next step is
- MQTT
- Bluetooth socket
- ……
Last , If you read this article , Yes Wifi P2P And cross platform communication research , Welcome to exchange .
Reference resources
In depth understanding of Wi-Fi P2P
Android 10.0 Change of relevant authority （ Use WiFi Direct Positioning has to be turned on ）
WiFi and AP Research on coexistence scheme --Android8.0
Android Realize file transfer without network 1
Android Realize file transfer without network 2（wifi hotspot ）
Mobile phone end to end direct connection technology
Android There is no network transmission file WifiP2P
Android Wi-Fi Display（Miracast） Introduce （ The fourth of them Miracast Topology map application scenarios are useful ）
